Switching mode power supplies are being increasingly used in many domestic and industrial applications. Apparatus such as televisions or computer monitors operate in one of a number of states or modes. For example, a first “off” mode occurs when there is no power being supplied to the apparatus; a second “on” mode occurs when the device is switched on and operating normally; and a third mode, referred to as a “standby mode”, occurs when the device is to remain powered but with reduced functions and reduced power consumption. In the case that the apparatus is a television, for example, the standby mode may, for example, be a mode in which the television is not displaying a picture or producing sound, but certain circuitry in the television remains powered so that, if the “on” button of the remote control is pressed the television will return to the “on” mode.
SMPS are implemented by supplying a regulated power supply to a the primary side of a transformer in series with a transistor. The secondary side of the transformer is connected to the apparatus (“load”). Switching of the transistor (usually, but not exclusively, switching off of the transistor; so called “fly-back” operation) causes variations in the current through the transformer, resulting in an output power on the secondary side of the transistor. The secondary side of the transformer is connected via a smoothing circuit to the apparatus to be powered. The average number of switching operations per unit time, and the current caused to flow in the transistor in each switching operation, together determine the average power transmitted to the apparatus. The main advantage of SMPS in comparison to conventional power supplies built up by means of linear regulators is their high efficiency at full load.
However, when the load decreases and the switching cycle remains the same, the efficiency of the SMPS decreases tremendously, since power losses are almost entirely due to the switching losses, which in turn are almost exactly proportional to the number of switching operations the transistor performs. A known solution to this problem is to reduce the number of switching operations per unit time as the load falls, such that the average number of switching operations is sufficient to supply the load. Since the number of switching operations is reduced, the switching losses decrease as the load is reduced.
There are several known methods for controlling the timing of the switching operations.
One solution is “frequency reduction”, in which in a given mode the switching operations on the transistor are periodic with a frequency substantially proportional to the power to be supplied to the load in that mode. Thus, in modes for which the power consumption of the load is low, the frequency of the switching operation is low, and thus the switching losses are low. Such a solution is described for example in the document “Data sheet TEA 1507”, published by Philips on 5 Dec. 2000. A disadvantage of this technique is that if the frequency of the switching operations decreases into the audible range, an audible noise is generated by the transformer.
Another solution is to maintain the frequency of the switching operations at the same value irrespective of whether the device is operating in high or low power mode, but in the low power mode to interrupt the switching operations. Thus, in this “burst mode” there are “bursts” (“frames”) of high frequency power pulses separated by periods in which there are no power pulses at all. The average power transmitted thus depends upon the proportion of the operation of the SMPS for which the bursts are transmitted. Such techniques too are described in the TEA1507 document. U.S. Pat. No. 6,392,06 also describes such a concept. The burst mode is entered by a signal generated from the secondary side of the transformer, and transmitted to the primary side by an optocoupler. Once, the burst mode is entered, the timing of the bursts is determined by a measurement of a voltage (“undervoltage”) on the primary side. This is known as “undervoltage lockout”. A disadvantage of this technique is that if the load rises during one of the periods between bursts then the circuit cannot react until the next burst is reached.
Another known technique is to control the burst mode based on a Vcc signal derived from a winding on the transformer. This is employed in the FS6Series of Fairchild (see, for example, application note 4116 published by Fairchild Semiconductor Corporation). The burst mode is entered if a feedback signal obtained from the secondary side of the transformer is kept below a certain level. The disadvantage with this is that the control is mainly taken over by the Vcc and therefore not directly load dependent.
Another known technique, employed in U.S. Pat. No. 6,385,061B1 and in the NCP1203 system of Semiconductor Component Industries LLC, is to start the burst mode when the load is below a certain level. The disadvantage of this concept is that there is no hysteresis implemented between the normal operation mode and the burst mode. Therefore, high frequency turning on and off of the burst mode (high frequency “bursting”) can occur if the changes around this level become small, and this too can lead to a disadvantageous audible noise.